The 3rd-Generation Partnership Project (3GPP) is continuing development of the specifications for the Universal Terrestrial Radio Access Network (UTRAN). More particularly, work is ongoing to improve the end-user experience and performance in Release 11 of those specifications. These efforts include work to improve end-user experience and system performance in the CELL_FACH state.
CELL_FACH is a Radio Resource Control (RRC) state in which the end-user terminal (user equipment, or UE, in 3GPP terminology) is known on cell level (i.e., it has a cell ID) and has a layer 2 connection, but has no dedicated physical layer resource. Instead, the UE in CELL_FACH state must share common physical layer resources with other users in CELL_FACH state.
The Enhanced Dedicated Channel (E-DCH), which is an uplink packet-access channel, can be deployed so that it is may be used by UEs in CELL_FACH state. More usually, E-DCH is used as a dedicated channel in CELL_DCH state, in which case a separate resource is allocated for each user. When E-DCH is used in CELL_FACH state, however, the system uses a pool of E-DCH resources that can each be temporarily assigned to a UE in CELL_FACH state.
This common pool of E-DCH resources is referred to herein as “common E-DCH resources.” E-DCH resources are normally managed by the Radio Network Controller (RNC), but the pool of common E-DCH resources is instead managed by the NodeB (3GPP terminology for a base station.) Configuration data specifying the E-DCH configurations are broadcasted to UEs in the cell.
FIG. 1 illustrates common E-DCH transmission in CELL_FACH state. Shown at the top is the Primary Physical Common Control Channel (P-CCPCH), a downlink physical channel that carries the broadcast control channel (BCH), which in turn carries system- and cell-specific information for UEs, such as indicators that specify which uplink scrambling codes are to be used. The P-CCPCH also serves as a timing reference for all physical channels.
The next channel illustrated in FIG. 1 is the Acquisition Indicator Channel (AICH). This physical channel is used to carry Acquisition Indicators (AIs), which correspond to preamble signatures transmitted by UEs and which are used by the NodeB to acknowledge the receipt of Physical Random Access Channel (PRACH) transmissions by UEs. The AICH can simultaneously acknowledge up to 16 PRACH preambles.
As shown in the next line of FIG. 1, labeled “RACH,” the procedure to access the common E-DCH channel in CELL_FACH begins in the same way as a Release-99 Random Access Channel (RACH), i.e., with preamble power ramping using randomly selected preamble signatures. In the illustrated scenario, the UE transmits a preamble in slots #0 and #3, with the second transmission having a higher power level. Having detected the second preamble transmission by the UE, the NodeB acknowledges the UE's PRACH transmission with an AICH sequence, in slot #6. It also informs the UE which common E-DCH resource it has assigned to the UE. The UE can then use the E-DCH resource, as shown in FIG. 1, beginning with slot #7. Also shown in FIG. 1 are the Enhanced Absolute Grant Channel (E-AGCH) and the Fractional Dedicated Physical Channel (F-DPCH), which are downlink channels used to control uplink transmissions. Not shown are the Enhanced Relative Grant Channel (E-RGCH) and the Enhanced Hybrid-ARQ Indicator Channel (E-HICH), which are additional downlink channels for controlling the uplink.
A common E-DCH resource is defined as a particular combination of the following: an uplink scrambling code; an E-DCH Radio Network Temporary Identifier (E-RNTI); an F-DPCH code and timing offset; E-AGCH/E-RGCH/E-HICH codes and signatures; and parameters for use by the UE in uplink High-Speed Dedicated Physical Control Channel (HS-DPCCH) transmissions, such as power offsets and Channel Quality Report configuration information.
As of Release 10 of the 3GPP standards, the CELL_FACH state is commonly used to provide an efficient use of radio resources for UEs when data is arriving in bursts, with longer idle periods in between. The goals include both an efficient use of the UE's limited battery resources, as well as an efficient use of the network's radio resources. Ideally, an UE should be inactive between the bursts but still be capable of swiftly moving into an active state when there are packets to send or receive. For this kind of on-off type traffic patterns, the connection set-up latency and signaling load has a significant impact both on the preservation of the device battery and on the transmission quality perceived by the end user. In dormant periods, UEs are either sent to Idle state or are set to use configured Discontinuous Receive (DRX) schemes, to save battery.
Information specifying E-DCH resource configurations is broadcasted to UEs using SIB 5, which a system information block sent over the BCH. Some of the broadcasted parameters, such as the Transmission Time Interval (TTI), are common for all common E-DCH resources.
The specifications for E-DCH as of Release 10 of the 3GPP specifications are rather rigid and do not allow flexible configurations. One example is the Transmission Time Interval (TTI) for common E-DCH resources. Currently, two different TTIs may be configured: either 10-millisecond TTI or 2-millisecond TTI. However, for coverage reasons, the network is likely to have some common E-DCH resources configured with 10-millisecond TTI. As specified today, this implies that all resources must the same TTI. However, UEs in good radio conditions, e.g., in so-called hot spots, could make good use of common E-DCH resource with a shorter TTI, i.e. 2 milliseconds. A shorter TTI improves both uplink throughput and network capacity, since each resource is occupied for less time. Concurrent deployment of 2-millisecond and 10-millisecond TTI will thus introduce improvements and provide the network with the flexibility to make an effective and optimal utilization of the common E-DCH resources.
Another issue with current specifications is that E-DCH operation with 2-millisecond TTI does not support per-HARQ activation and de-activation for those common E-DCH resources. This feature is specified for CELL_DCH state. Per-HARQ-process activation/de-activation allows the network to keep a much closer control over the resources and the interference level. Per-HARQ-process activation/de-activation is performed by the NodeB. The problem resides in the fact that the Node-B does not have an explicit indication of the UE release. If per-HARQ-process activation/de-activation were to be standardized, there could be an inter-operability issue. If the Node-B sends an HARQ activation/de-activation order to a pre-Release-11 UE, i.e., a UE that is not compliant to Release 11 specifications, that UE will not act on that order according the specifications.
The utilization of common E-DCH resources may be inefficient for certain types of traffic, especially for very small packets like TCP ACKs. This inefficiency arises from the fact that the time from initial RACH preamble to active transition (the actual time depends on the system configuration) may be prefaced by DPCCH transmission, as well as that the relative overhead for a single, small payload transmission is rather large. In other words, the total resources used for control channel and signaling are relatively high, compared to very small transmissions like a TCP ACK.
Still other inefficiencies arise from the fact that the common E-DCH resource used by a UE is not released until all the HARQ processes have been acknowledged. This last issue means that common E-DCH resources are allocated for relatively long periods for the time actually needed for the transmission. From the network point of view, the network will wait at least one HARQ Round-Trip Time (RTT) from the last acknowledged HARQ process before it re-uses the resource for another UE. In practice, this means that the common E-DCH resource is occupied longer than a Resource-99 PRACH resource could be, given that only one packet is transmitted. Furthermore, in high load scenarios, the AICH NACK rate will increase due to the lack of common E-DCH resources.
To address some of these problems, a fallback approach is being standardized. With this approach, a Release-99 RACH process is used to access the network, even by Release-11 capable UEs, instead of using one of the common E-DCH resources. However, a similar problem to one of those noted above arises in the deployment of this fallback approach. Namely, the Node-B does not know the UE release. If the network wants to explicitly indicate to the UE to fall back to RACH using already specified Release-99 procedures, there will be interoperability issues, since pre-Release-11 UEs will not recognize and act on those orders. Additionally if new procedures are standardized, the NodeB needs to know which UEs are able to handle them.
In brief, the various significant issues with the use of E-DCH in CELL_FACH state include the following: how to control and configure the usage of the concurrent TTI configurations; individual UE capabilities are unknown before contention resolution; and network control is needed to handle overall interference, hardware resources, HARQ operation point, cell coverage, scheduling, etc.